Modern gas lasers are designed to deliver more and more pulse energy at higher and higher repetition rates. Typical power levels now range from 50 W to greater than 150 W. A wide range of applications has arisen for lasers, which require the laser to operate for long periods of time between maintenance.
Gas lasers use a pair of optics (a full mirror at one end and a partial mirror at the other end) to seal the discharge chamber within which the lasing takes place. The output beam is emitted from the partially mirrored optic.
The optics of a gas laser are subject to degradation when used for prolonged periods of time. Also, the gas in the cavity of the laser generates impurities during operation. These impurities ultimately "poison" the laser gas, which must eventually be replaced. This deterioration of the gas may contribute to the degradation of the optics.
The optics are subjected to radiation from the laser beam, and this radiation is particularly intense when the laser is operating in the ultraviolet frequency range, e.g., excimer lasers. Also, in the case of an excimer laser the optics may be attacked by corrosive halogen gases. Nevertheless, basically the same problem of the deterioration of the optics arises at least to some extent in all gas lasers, not only excimer lasers. For example, dust and dirt can find their way onto the inner surfaces of the optics causing etching of such inner surfaces. Also damage centres may form in the bulk of the material. All the factors described above result in an increased absorption of energy by one or both of the optics of any gas laser.
As a result, the reflection and transmission characteristics of the optics decrease with time. Consequently, the output power of the laser is reduced, for which reason one or both of the optics will eventually need to be removed for cleaning or replacement. The interval between cleaning depends upon the operating conditions of the laser, the amount of dust in the laser chamber, and the particular lasing medium employed.